1. Field of the Invention
The present invention relates to an all-solid-state cell utilizing a combination of an electrode active material and a solid electrolyte material.
2. Description of the Related Art
In recent years, with the advancement of portable devices such as personal computers and mobile phones, there has been rapidly increasing demand for batteries usable as a power source thereof. In cells of the batteries for the purposes, a liquid electrolyte (an electrolytic solution) containing a combustible organic diluent solvent has been used as an ion transfer medium. The cell using such an electrolytic solution can cause problems of solution leakage, ignition, explosion, etc.
In view of solving the problems, all-solid-state cells, which use a solid electrolyte instead of the liquid electrolyte and contain only solid components to ensure intrinsic safety, have been developing. The all-solid-state cell contains a sintered ceramic as the solid electrolyte, and thereby does not cause the problems of ignition and liquid leakage, and is hardly deteriorated in battery performance by corrosion. Particularly all-solid-state lithium secondary cells can achieve a high energy density easily, and thus have been actively studied in various fields (see, for example, Japanese Laid-Open Patent Publication Nos. 2000-311710 and 2005-063958, Yusuke Fukushima and four others, “Fabrication of electrode-electrolyte interface in all-solid-state cell lithium batteries using the thermal softening adhesion behavior of Li2S—P2S5 glass electrolytes”, Lecture Summary of Chemical Battery Material Association Meeting, Vol. 9th, Pages 51-52, issued on Jun. 11, 2007)
Japanese Laid-Open Patent Publication No. 2005-063958 discloses a thin-film, solid, lithium ion secondary cell. The secondary cell described in Japanese Laid-Open Patent Publication No. 2005-063958 is a bendable thin-film cell having a flexible solid electrolyte and thin layers of positive and negative electrode active materials sputtered thereon. The electrodes of the cell have to be thin, and the amounts of the electrode active materials are limited. Thus, the cell is disadvantageous in that it is difficult to achieve a high capacity.
The article of Fukushima et al. reports formation of an electrode-electrolyte interface of a complex of a glass electrolyte and an electrode active material, utilizing softening fusion of the glass electrolyte. In this report, it is described that the resistance between electrolyte particles is effectively lowered due to the fusion of the glass electrolyte, and further a heterophase is not formed in a reaction between the electrolyte and the active material.
However, an all-solid-state cell having positive and negative electrodes is not described in this report, and it is unclear whether the reaction resistance can be lowered in the electrolyte-electrode active material interface. Further the relation between the electric properties and the fact that the heterophase is not formed is not specifically described, and the charge-discharge ability of the all-solid-state cell is unknown. Furthermore, the electrolyte used in this report is a sulfide, which is expected to be unstable in the atmosphere (air). The electrolyte may generate a toxic gas when brought into contact with the air due to breakage or the like. Thus, this technology is disadvantageous in safety.
The internal resistance of a cell is partly due to an interface between an electrode active material and an electrolyte. The resistance against transfer of electrons and Li ions through the interface during charge and discharge is hereinafter referred to as the interface reaction resistance. The present invention relates to a technology for lowering the interface reaction resistance in an all-solid-state cell system using a solid electrolyte.
For example, in the conventional lithium ion cell using the electrolytic solution, the electrolyte is a liquid containing an organic solvent, though the electrode active material is a solid. Therefore, the electrolyte can readily penetrate between particles of the electrode active material to form an electrolyte network in the electrode layers, resulting in a low interface reaction resistance.
In terms of the interface reaction resistance according to the present invention, a reaction resistance per unit area of connected particles largely depends on the combination of the active material and the electrolyte to be used. As the connected area between the particles is increased, the interface reaction resistance of the entire cell is lowered and the internal resistance is lowered such that resistances are parallel-connected in an equivalent circuit. Thus, the interface reaction resistance between the electrolyte and the active material can be lowered by (1) selecting the material combination in view of smoothly transferring the Li ions and (2) increasing the connection interface area between the electrolyte and the active material per an electrode capacity.
In the present invention, a combination of an electrode active material and a solid electrolyte containing a common polyanion or a combination of an electrode active material and a solid electrolyte of phosphate compounds is used in view of the process of (1), and a solid electrolyte is mixed with an electrode active material to form a network in an electrode layer, whereby the connection interface area between the electrode active material and the solid electrolyte is remarkably increased to lower the interface reaction resistance in view of the process of (2).
Japanese Laid-Open Patent Publication No. 2000-311710 discloses a solid electrolyte cell containing a solid electrolyte material of an inorganic oxide, which forms a three-dimensional network between particles of an electrode active material. Thus, the inventors have selected the combination of the phosphate compounds containing a common polyanion as the combination of the electrode active material and the solid electrolyte suitable for smoothly transferring the Li ions, and have produced an all-solid-state cell having electrodes containing the solid electrolyte between the electrode active material particles. However, because the solid electrolyte was fired in the state of a mixture with the electrode active material in the electrode layer, the electrolyte was reacted with the active material, so that reduction in the peak intensity of the active material and formation of a heterophase were found in an XRD (X-ray diffraction) observation. The active material in this state was subjected to a charge-discharge ability measurement using an ideal system containing an electrolytic solution. As a result, the charge-discharge capacity of the active material was extremely reduced, and the active material was incapable of charge and discharge at its original theoretical capacity. Thus, the capacity of the electrode active material was lowered.
Then, the inventors have lowered the firing temperature to prevent the reaction between the electrode active material and the solid electrolyte. However, the solid electrolyte particles were not sufficiently sintered, the particle boundary resistance between the solid electrolyte particles was increased, and the connection interface area between the electrode active material and the solid electrolyte was not increased. As a result, both the particle boundary resistance of the solid electrolyte and the interface reaction resistance of the electrode active material and the solid electrolyte could not be lowered, whereby the resultant all-solid-state cell had no charge-discharge capacity (no charge-discharge ability).